Magnetron construction



Feb. 3, 1970 B. v. VALLES 3,493,810

MAGNETRON CONSTRUCTION Filed Feb. 16, 1968 2 Sheets-Sheet 1 Feb. 3,1970l B. v, v'ALLEs MAGNETRoNcoNsTRUcTIN 2 Sheets-Sheet 2 Filed Feb. 16,1968 United States Patent O MAGNETRON CONSTRUCTION Benjamin V. Valles,San Jose, Calif., assignor to Litton Precision Products, Inc., SanCarlos, Calif., a corporation of Delaware Filed Feb. 16, 1968, Ser. No.706,119

Int. Cl. H01j 25/50 U.S. Cl. S-39.51 13 Claims ABSTRACT 0F THEDISCLOSURE An improved magnetron construction is provided which includesan elongated evacuated container housing in a vacuum, both theconventional emissive cathode and the resonant cavity containing anodesurrounding the cathode and separated by an annular space; a top endportion of the elongated container includes a microwave window throughwhich the microwave energy is therefrom extracted; and means areprovided for conventionally establishing a crossed electric and magneticfield within the annular space between the anode and cathode within thecontainer. The means for establishing the magnetic field within theannular space includes, external of the container, a pair of spacedparallel substantially flat rectangular ferromagnetic pole pieces ofapproximately equal dimensions with each having a central passage. Thet-op end portion of the elongated container protrudes through thecentral passage in one of these pole pieces, a bottom end portion of theelongated container protrudes through the central opening in the otherpole piece, and the central portion of the container is accordinglylocated between the two pole pieces. Cylindrical permanent magnetsextend -between the confronting ends of the spaced pole pieces to theiight and the left of the central passages to provide sources ofmagnetic flux. The improved construction encompasses a spaced pluralityof horizontal cooling fins located Ibetween and aligned substantiallyparallel with the at surfaces of the pole pieces. A central passage andtwo spaced passages separated by the central passage are located in thesubstantially at major surface area of each cooling fin. In each cooling1in the central passage is located so that it surrounds and is directlyor indirectly in thermally conductive contact with the outer peripheryof a portion of the elongated evacuated container, while each of theother passages is aligned with and surrounds a respective one of the twocylindrical magnets. Thus, a portion of each of the cooling fins extendsoutwardly from the magnets to form a grill or fence-like barrier infront of each magnet, and, accordingly, this prevents direct contactbetween the permanent magnets and any relatively large ferromagneticobject. Notably, each -cooling fin has a bent over lip portion alongeach of two opposed edges. Each lip portion extends from the cooling nfrom which it is formed, preferably to the next adjacent cooling fin toat least partially form an air duct or channel. The lip portions of eachcooling lin are preferably aligned in the same direction. However, thebottom cooling fin has the lip portions extending in the oppositedirection. The central passage of each cooling n preferably includes aslightly flared rim or lip portion surrounding the passage, whichextends from the fin surface in the same direction as the lip portions.Moreover, a sleeve-like conductive body of the same material as thecooling iin advantageously surrounds the outer periphery of a portion ofthe elongated evacuated container and is thus interposed between thefins and the container to enhance heat conduction between the containerand the cooling fins.

This invention relates to a magnetron, and more parlCC ticularly, to anovel construction of a magnetron in which the cooling fins protect thepermanent magnets from undesired physical contact with large objects.

The magnetron is a well known device useful for the generation of highfrequency electromagnetic energy and is in the group of electromagneticdevices which rely for operation upon the eifects of crossed electricand magnetic elds. In the magnetron, electrons are emitted from anelectron emissive cathode into an annular space between the cathode anda circumferentially surrounding anode in a high vacuum environment of anevacuated container. The anode contains thereabout a plurality ofresonant cavities commonly formed by vanes which protrude from acylindrical portion of the anode into the annular region; and, the meansare provided for establishing a crossed electric and magnetic eld inthis annular region; the electric field being conventionally establishedradially between the cathode and the anode by the geometry and locationof the elements and the application of suitable electric potentials, andthe magnetic field being established perpendicular to such electricfield suitably by arrangement of elements and the use of externallyconnected magnets and magnetic circuits.

Acting under the influence of the electric and magnetic field,electronic interaction and oscillation occurs within the annular spacebetween the electrons emitted from the cathode and the electric fieldsestablished within the resonant cavities to generate electricaloscillations of a very high frequency. The particular details and theoryof operation of such devices can be had by reference to availableliterature. The high frequency or microwave, as variously termed, energygenerated within the container is conducted from one or more of theanode cavities to an exit or microwave window which maintains theevacuated cavity sealed, Ibut which permits the passage of the microwaveenergy. f

At this point it is best to clarify the nomenclature used to describethe magnetron. Some persons denote the component consisting of the anodeand cathode in the evacuated container, while they describe the externalmagnets, pole pieces, cooling fins, and other ancillary elements thatare used either as a necessary element or a proper element to supportthe generation of electromagnetic energy within the container as thefittings Others, however, denote the entire assembly of sealed evacuatedcontainer, pole pieces, magnets, cooling fins, and other necessary orproper elements as the magnetron, and denote rather ambiguously thesealed evacuated container portion thereof also as the magnetron.

Accordingly, to avoid this apparent difficulty of nomenclature and toavoid confusion for purposes of this patent application, it is believedbest to refer to the entire assembly of elements as a magnetron ormagnetron device and to denote the evacuated container portion whichcontains the anode and cathode in a vacuum environment only in broadgeometrical terms. Suitable terminology as is available for the otherexternal necessary or proper ancillary elements thereof is adoptedherein or is otherwise clearly defined.

As is conventional with other energy conversion devices, the generationof heat occurs as an undesirable side effect which may render themagnetron less efficient or inoperative. Hence, the heat generatedduring operation is normally removed from the container. Typically, theheat is generated in the anode and is conducted away by passing throughthe walls of the evacuated container to the outside thereof for exposureto a cooling medium. Conventionally, cooling fins are attached to thecontainer to receive and dissipate the heat. The cooling tins are thinmetal sheets of a large area and expose the heat to a large area ofcooling medium. The cooling tin with the cooling medium merely acts as aheat sink and by conduction and convection transfers heat to a coolingmedium such as oil or ambient air. More typically, with a blower air isforced against the cooling fins to more rapidly dissipate such heat.

Conventionally, cooling fins may be placed in contact with the walls ofthe evacuated container and arranged geometrically either vertically orhorizontally relative to the axis of the evacuated container. The sizeof the cooling fins is dictated and limited to an extent either by theavailable space within which it must fit, taking into account the sizeand shape of the other elements of the magnetron, such as the magnetsand the pole pieces, the size of the air blower, and the amount of heatto be dissipated.

In one highly successful and efficient magnetron manufactured by LittonIndustries, Electron Tube Division of San Carlos, California, under thedesignation I4-5001, a particularly eilicient construction for amagnetron is used. Basically, in that construction two rectangularshaped pole pieces substantially of the same dimensions are spaced apartparallel, and each contains a central passage or opening. An elongatedevacuated container which encloses the cathode and anode exists betweenand through the central passages and substantially at right angles tothe pole pieces. The front end portion of the evacuated container withthe microwave window extends through the central passage of one polepiece, and a bottom end portion of the container extends through thecentral passage in the second pole piece. Surrounding the evacuatedcontainer and in physical Contact therewith is an apparently widecylindrical body. Close examination shows that this body consists of aplurality of thin individual vertically arranged cooling fins strappedtogether to hold them in place. Each cooling fin includes two lipportions on opposed edges and extends radially outwardly from theperiphery of the elongated container portion of the magnetron. The innerlip portion of each fin abuts the outer wall of and is parallel with theaxis of the elongated evacuated container. A cylindrical shapedpermanent magnet is located between each of the two confronting faces atthe ends of the pole pieces and extends between those faces. In onemodel of this magnetron each of these cylindrical magnets is surroundedby a cardboard cylinder of appropriate thickness. In another, analuminum shield is spaced from and partially encloses each magnet. Themagnets and container are clamped into place between the pole pieces bybolts which extend between the pole pieces. A pictorial representationof such magnetron is found in U.S. Patent D. 208,861.

The cardboard cylinder or aluminum shield referred to prevents anyforeign object of ferromagnetic material from being placed into directcontact with the magnetron. As is known, when large ferromagnetic (iron)objects are placed into contact with permanent magnets, they shunt someof the magnetic ux from the magnet and, when removed, the magnetic fluxemanating from the magnet is altered, the magnet s partiallydemagnetized and the intensity of the flux available from the magnet isreduced. With magnetrons or other crossed field devices, it isabsolutely necessary that such foreign objects be maintained away fromthe permanent magnets. Inasmuch as the operation of a magnetron devicedepends upon the maintenance of the critical design values of theelectric and magnetic fields within the annular space between thecathode and anode, any departure of the magnetic fields from suchcritical levels renders the magnetron wholly or partially inoperative.Accordingly, great care is taken to prevent the demagnetization of themagnets when the magnetron is handled or is otherwise in use, whichheretofore included the alternatives of an aluminum surround orcardboard wrapping. However, it is apparent that such preventativesrequire additional and different types of manufacturing processes andprocedures than is used for the assembly of the other elements of themagnetron, which necessarily add to its cost.

Therefore, it is an Object of the invention t9 provide a magnetron inwhich the likelihood of accidental demagnetization of the permanentmagnets is reduced;

It is a further object of the invention to prevent direct contactbetween the permanent magnets of a magnetron and a foreign ferromagneticobject; and

It is a still further object of the invention to provide a magnetron inwhich the permanent magnets cannot contact any large foreignferromagnetic objects or be partially demagnetized thereby, even thoughspecial cardboard sleeves are not used; and

It is an additional object of this invention to provide a magnetronwhich requires no separate elements or assembly operations for providingelements which prevent contact between the magnets and large foreignmagnetic objects.

The foregoing and other objects and advantages of the invention arebetter understood by considering the following detailed descriptiontogether with the accompanying figures of the drawings, in which:

FIGURE l illustrates a partially exploded and cutaway perspective viewof a novel magnetron Construction embodying the invention;

FIGURE 2 illustrates the construction of a cooling fin used in theembodiment of the invention shown in FIG- URE l and in FIGURE 3;

FIGURE 3 illustrates in perspective a novel magnetron embodying theinvention; and,

FIGURE 4 is a side view of the embodiment of FIG- URE 3.

Briefly stated7 the invention is characterized by a construction of amagnetron in which a plurality of horizontally arranged parallel coolingfins in addition to being in thermally conductive contact with theelongated evacuated container surround two spaced permanent magnets, andthus present a grill or fence-like barrier to foreign ferromagneticobjects.

In order to permit a less obstructed examination of the detail in thisembodiment of the invention, portions of elements in FIGURE 1 arecutaway and others are exploded. The magnetron includes `a sealedevacuated container portion indicated generally as one which includesinternally (not illustrated) the conventional cathode anode, and theconcentrically spaced annular anode to form therebetween an annularspace, Such anode conventionally contains a plurality of spaced vaneswhich project from the anode walls to within a predetermined distance ofthe cylindrical cathode to form an annular space. This space may betermed the interaction region. Between each of the respective vanes aradio frequency cavity resonant at the particular design frequency whichthe magnetron generates is formed. Also included therein is a filamentor heater, which extend externally of the container at terminal leads 3and 5, provide electrical connections which extend into container 1 tothe heater winding for connection to a suitable source of voltage andcurrent.

The internal constuction of the magnetron elements looated withincontainer 1 and container 1 itself may assume any conventionalconstruction and, as presented here, is substantially the same as thatheretofor designed, manufactured, and sold by Litton Industries,Electron Tube Division, San Carlos, California, under the designationL-500l.

A top end portion of container 1 contains a glass bulb or microwavewindow 7 which forms the exit for microwave energy generated in theannular region between the anode and cathode within container 1. As isconventional, such microwave energy is coupled from one of the resonantanode cavities to the microwave window 7 by means of a conductor 9extending therebetween. The bottom portion of container 1 includes twoprotruding electrical terminals or leads, 3 and 5, which are insulatedfrom each other and which are connected internally of Container 1 to afilament or heater winding.

A circular collar 11, constructed of ferromagnetic man terial isillustrated with a portion cutaway. This collar fits the outer peripheryof the container and a passage in the pole pieces, hereafter described.Collar 11 forms part of the magnetic circuit and contacts a portion ofcontainer 1 that is constructed of ferromagnetic material. Elementsinternal of container 1 complete the magnetic circuit to the top of theinteraction region. Another circular collar of ferromagnetic material isillustrated cutaway and exploded. Collar 13 also forms part of themagnetic circuit.

A cylindrical aluminum sleeve 15, illustrated with a portion cutaway, ismounted to container 1 by any conventional process, such as pressing onor shrink fitting. This ensures a strong mechanical connection betweenthe cylindrical copper walls of container 1 and the inner walls ofsleeve 15. A pair of fiat rectangular pole pieces 17 and 19 areprovided. Pole piece 17 is illustrated with a portion cutaway to permita view of other elements hereinafter described. Upper pole piece 17 isconstructed of ferromagnetic material and contains a circular opening orpassage 21 therethrough. As is apparent, the upper portion of container1, and, in particular, the microwave window 7 protrudes through thisopening with the pole piece 17 abutting collar 11. A sutiable wire meshseal 12, illustrated with a portion cutaway, fills the annular groovebetween window 7 and the edge of passage 21.

Likewise, the bottom pole piece 19 includes a central opening or passage23 through which the bottom portion of container 1, including leads 3and 5, upon assembly, protrudes. The bottom collar 13, upon assembly,tightly abuts both pole piece 19 and a wall of container 1 that is offerromagnetic material. A first cylindrical magnet 25, suitably ofAlnico V, is located between the confronting ends of pole pieces 17 and19 on one side of central passages 21 and 23. Likewise, anothercylindrical shaped permanent magnet 27 is located between the remainingtwo confronting ends of pole pieces 17 and 19 on the opposite sides ofthe central passages 21 and 23. The permanent magnets are polarized inthe same direction. As assembled, finally pole piece 17 abuts the bottomof both magnets 25 and 27 and a shoulder portion of collar 13. Collar 13abuts a ferromagnetic wall portion of container 1. Thus, the externalmagnetic path is completed from one end of magnet 25 through pole piece21, the top collar 11 and a ferromagnetic wall portion of container 1,through the magnetic path internal of container 1, a bottomferromagnetic wall portion, collar 13 and pole piece 19, back to theother side of the magnet. A similar magnetic flux path extends from thetop side of permanent magnet 27 through shoulder 11, the container 1wall, intemal elements, bottom wall of the container, bottom collar 13,pole piece 19, returning to the 'bottom of the magnet 27.

As is conventional, elements internal of container 1 complete the pathfor magnetic flux from the container Walls to positions above and belowthe aforecited interaction region. A plurality of cooling fins 29,illustrated with a large portion -cut away, are provided. Each has amajor substantially lfiat surface area 31 which is preferably arrangedsubstantially parallel to the flat surface of the pole pieces andsubstantially perpendicular to the axis of the cylindrical permanentmagnets 25 and 27 and the axis of elongated container 1.

Reference is briey made to FIGURE 2 which shows in greater detail theconstruction of a cooling n 29 used in the embodiment of the inventiondisclosed in FIG- URES 1 and 3. In order to properly show the lipportion, hereinafter described, cooling iin 29 is inverted from theposition in which it appears in both FIGURES l and 3.

Each of the cooling fins contains a central passage 35 which issubstantially the same inner diameter as the outer diameter of sleeve15. A second passage 37 and a third passage 39 are located on oppositesides of the central passage 35. The diameter of the latter passages isslightly larger than the diameter of the respective permanent magnets 25and 27. Geometrically the locations of the three openings in the coolingn are aligned with and correspond to the 4spacing and relative positionof the sleeve surrounded elongated evacuated container 1 and permanentmagnets 25 and 27, illustrated in FIGURE l.

As is apparent, since passages 37 and 39 are formed in the major surfacearea 31 of the cooling fin and surround the respective magnets, aportion of the cooling iin is situated between the edges of each ofthose passages and the edge of the cooling fin. Surrounding centralopening 35 is a rim or lip portion 41 which is formed from and extendsfrom the major surface area of the cooling fin. The depth of lip 41 maybe substantially equal to the desired distance or spacing desired in thefinal assembly,

between adjacent cooling fins or less as desired. With the former depththe lip will function additionally to space adjacent cooling fins.

The inner diameter of lip portion 41 is substantially the same as theouter diameter of sleeve 15. Preferably, the diameter of the lip portionis slightly smaller than the diameter of central passage 35 at thesurface 31 of the cooling fin in order to provide a slight inward are.The lip portion 41 compressively surrounds and engages sleeve 15 to forma tight fit, as assembled. Moreover, because the depth of lip 41 isgreater than the thickness of the cooling fin, a substantially betterthermal contact is effected between sleeve 15 and the major surface area31 of the cooling fin than would be available if only the contactingportion was of the same thickness as cooling fin 29.

In addition, as shown in FIGURE 2, each cooling n includes two edge lipportions 43 and 45 on opposed edges of the cooling fin. The lip portionis formed by bending over a portion of the cooling n substantially atright angles to major surface 31, and is preferably of a depthsubstantially the same as the distance desired for the adjacent coolingns in the assembled magnetron.

Referring again to FIGURE 1, it is apparent that each cooling fin isforced onto sleeve 15 and slid along the sleeve until the surface ofthat cooling iin preferably abuts the end of a lip portion of apreviously mounted cooling fin. Ideally in this final position, lipportions 43 and 45 of a preceding cooling fin contact the edges of thecooling fin being mounted. This forms an air duct or channel 47. Thebottommost cooling fin 29 is preferably inverted.

Accordingly, cooling fins 29 are assembled onto sleeve 15 and bottompole piece is mounted into place. A plurality of rivets or bolts 49extend between pole pieces 17 and 19 passing through opposite polestherein to clamp the permanent magnets 25 and 27 and the evacuatedcontainer 1 therebetween.

Reference is now made to FIGURE 3 which shows the preferred embodimentof the invention in assembled form to avoid confusion and simplify thisexplanation like elements of like figures identically labeled. In thisView, a portion of sealed evacuated container 1 is visible, themicrowave window 7 is shown, wire mesh seal 12, pole pieces 17 and 19,and cooling fins 29 are clearly presented.

It is apparent that between the last and first cooling fins a pluralityof substantially identical air ducts such as 47 are formed. Moreover, itis clearly seen that passage 39 surrounds a portion of permanent magnet25 leaving a portion of the cooling fin surface protruding in front ofthe magnet. Not visible however, the other passage surrounds magnet 27.

As is further apparent, the protruding edges of the cooling fins 29together form a grill or fence-like barrier to objects having a largerdimension than the distance between any two adjacent cooling fins. Sincethe larger ferromagnetic objects which cause the greater demagnetizationof the permanent magnets, it is apparent that by a suitable choice ofspacing between adjacent cooling fins, contact l"between the permanentmagnets and such foreign objects is prevented.

lt is noted that the bottommost cooling lin 29 is preferably inverted;that is, it has its lip portions 43 and 45 and lip portion 41 extendingin a direction opposite to that on the other cooling ns vwhich forms anair duct or channel of a double thickness, and avoids the location of asharp edge at the bottom.

FIGURE 4 illustrates a side view of the embodiment of the inventiondescribed in FIGURE 3. Again to avoid confusion and simplify thisexplanation, like elements of like figures are identically labeled. Inthis view it is clear that lip portion 41 is in physical contact withsleeve. Thus, since metal sleeve l5 is in tight physical contact withthe walls of the elongated container, a good thermally conductivecontact is provided between the walls of the elongated container andcooling fins 29. While a sleeve 15 is physically interposed between thefins and the container walls, it may with suitable changes in dimensionof the elements be omitted, and the ns made to directly contact thecontainer. However, the disclosed arrangements are clearly preferred. Inaddition, air ducts, such as duct 47, formed by fins 29 are clearlyillustrated in this figure. In use, the embodiment of the improvedmagnetron, as is conventional with all other magnetrons, is connected toa conventional and suitable high voltage power supply and filamentcurrent supply. The filament supply is connected between terminals 3 and5, and the high voltage supply is connected (negative polarity) to oneof filament terminals and ground. ln the magnetron of the invention, theelectron emitting cathode is internally within container i connectedwith the heater winding and the anode is connected to the metallicportion of container 1 which is in turn grounded. This is entirelyconventional. Likewise, the electromagnetic energy generated withincontainer 1 is extracted therefrom via the microwave window 7 and bymeans of waveguide or other conventional transmission line istransmitted to other conventional components of a system which uses theelectromagnetic energy. Suitably such a system may be a conventional onein which the microwave energy is used to heat dielectric materials. Heatgenerated within container 1 is transmitted to the surfaces of coolingfins 29 where the heat is more rapidly dissipated, inasmuch as the heatis exposed to a cooling ambient, such as air, over a large number oflarge surfaces. Conventionally, to speed up the rate of heatdissipation, air is circulated through the formed air ducts between thecooling fins with a blower. The conventional function and use ofmagnetron elements is thus combined with the function of protecting thepermanent magnets by encasement in the cooling fins to provide animproved magnetron.

It is understood that the embodiment and description presented areintended to clearly illustrate the invention and are not intended in anyway to limit the invention, since numerous other equivalents suggestthemselves to those skilled in the art and which do not depart from thespirit and scope of the disclosed invention.

For example, one or more of the collars 13 and 11 may be made integralwith either the adjoining pole piece or with the adjoining magnetic wallportions of elongated container 1 by simply redesigning the geometry ofthe illustrated container or pole piece and machining the parts so thatthey properly match Moreover, while the illustrated embodiment shows aplurality of rivets 49 extending between pole pieces 17 and 19 topermanently clamp the assembled elements together, such means may bedeleted entirely. As an alternative, it is found that the magneticstrength of permanent magnets 25 and 27 alone are sufficient to retainthe ferromagnetic pole pieces and hence the other elements in position,although some slight amount of twisting movement is possible. However,to eliminate even this, small amounts of contact cement can be added tomore firmly join the magnets and pole pieces. A further departure fromthe illustrated details which is within the spirit and scope of theinvention is evident.

By suitable redesign of the elongated container 1, the aluminum sleeve15 may be deleted and the cooling fins may be fitted directly onto theouter cylindrical walls of container 1. In such instance the outer wallsof container portion 1 may have an annular portion extending from thebottom which surrounds portions of the length of the heater leads 3 andS to permit the use of the illustrated number of cooling fins.

Accordingly, it is to be expressly understood that the invention is tobe broadly construed within the spirit and scope of the appended claims.

What is claimed is:

l. A magnetron of the type which includes: an elongated evacuatedcontainer having a microwave window at a top end portion thereof,electrical terminals at a bottom end portion thereof, and, includinginternally thereof, an electron emissive cathode and a resonant cavitycontaining anode surrounding and spaced from said cathode; and magneticfield means for establishing a magnetic field within the space betweensaid anode and cathode within said elongated container; said magneticfield means further comprising, externally of said elo-ngated container:a pair of spaced pole pieces of ferromagnetic material, each having acentral passage therethrough; a first permanent magnet means yextendingbetween two confronting ends of said two spaced pole pieces to one sideof said central passages; a second permanent magnet means extendingbetween the remaining two confronting ends of said two spaced polepieces; and wherein said microwave window protrudes through a centralpassage of one of said pole pieces and said electrical terminalsprotrude through the central passage in the other one of said polepieces, the improvement comprising in combination: a plurality ofmetallic cooling fins located between said pole pieces and alignedsubstantially parallel thereto; and, wherein each of said cooling finscomprises: a major surface area; a central passage in said centralSurface area surrounding and being in thermally conductive contact withthe outer periphery of a portion o-f said elongated container; secondand third passages in said surface area spaced from the edges of saidmajor surface and spaced from each other on opposite sides of saidcentral passage; said second passage being aligned with and surroundingsaid first permanent magnet means; and, said third passage being alignedwith and surrounding said second permanent magnet means, whereby saidcooling fins form a grill-like barrier in front of each permanent magnetmeans to prevent physical contact between the magnet and most geometriesof foreign ferromagnetic objects.

2. The invention as defined in claim 1 wherein said thermally conductivecontact between `said fins and said container comprises: a metallicsleeve on its inner periphery surrounding and tightly fitting the outerperiphery of a portion of said elongated container and 0n its outerperiphery tightly engaging each of said cooling fins.

3. The invention as defined in claim 2 in which said metallic sleeve andsaid cooling fin comprise the same ma` terial.

4. The invention as defined in claim 2 wherein each of said cooling finsincludes bent over lip portions formed along each of two opposed edges,each of said lip portions being aligned at substantially a right angleto and extending in the same direction from said major surface of saidcooling fin.

5. The invention as defined in claim 4 wherein each said lip portionextends between the cooling fin from which it is formed to an adjacentcooling fin to form an air duct therebetween.

6. The invention as defined in claim 5 further comprising: a rim lipportion surrounding said central opening in each of said cooling fins toenhance thermal conduction between each said cooling fin and saidelongated container.

7. The invention as defined in claim 6 wherein each of said pole piecescomprises a fiat rectangular shape geometry of substantially similardimensions, and wherein each of said permanent magnet means comprises acylindrical shape geometry, and each of said passages in said coolingfins and pole pieces comprises a circular geometry.

8. The invention as defined in claim 6 wherein said rim lip portionextends substantially between the surface of the cooling fin from whichit is formed to the next adjacent cooling fin.

9. A magnetron device for the generation of high frequencyelectromagnetic energy of the type which includes: an elongatedevacuated container; an electron emissive cathode and a resonant cavitycontaining anode surrounding and spaced from said cathode enclosedwithin said container; a microwave Window for the extraction ofmicrowave energy located at a top end of said container; electricalterminals at a bottom end of said container; and, wherein a crossedelectric and magnetic field is to be established within the annularspace in said container between said anode and cathode; and, whereinmeans for establishing a magnetic field within said annular spaceincludes externally of said container: a pair of spaced substantially atpole pieces of ferromagnetic material of approximately equal length;each of said pole pieces having a central passage therethrough; a firstcylindrical permanent magnet means which extends between two confrontingends of said two spaced pole pieces at substantially right anglesthereto, a second cylindrical permanent magnet means which extendsbetween the remaining two confronting ends of said two spaced polepieces at substantially right angles thereto; and, wherein saidmicrowave window protrudes through a central opening in one of said polepieces, and said electrical terminals protrude through a central openingin the other one of said pole pieces; the improvement comprising incombination: a plurality of cooling ns located between said pole piecesand aligned substantially parallel with the flat surfaces of said polepieces, each of said cooling fins having a substantially flat majorsurface area, a central passage in said surface area surrounding andbeing in thermally conductive contact with the outer periphery of aportion of said evacuated container, and having second and thirdpassages in said surface area spaced from the edges of said majorsurface and spaced from each other on opposite sides of said centralpassage; said second passage being aligned with and surrounding saidfirst permanent magnet means, and said third passage being aligned withand surrounding said second permanent magnet means, whereby said coolingfins forms a grill-like barrier in front of each permanent magnet meansand prevent physical contact of the magnet with most geometries offerromagnetic objects.

10. The inventionas defined in claim 9 wherein each of said cooling finsincludes a bent over lip portion along each of two opposed edges, andwherein said lip portion is aligned at substantially a right angle tothe major surface of said cooling fin and substantially extends betweenthe major surface area thereof and an adjacent cooling fin to form anair duct therebetween.

11. The invention as defined in claim 10 wherein substantially all lipportions of all cooling fins face the same direction, and the lipportions of a bottom cooling fin faces an opposed direction.

12. The invention as defined in claim 11 wherein said central passage ofeach of said cooling fins includes a rim lip portion surrounding the-passage opening.

13. The invention as definedin claim 12 including a cylindrical metalsleeve of the same metal as said cooling fins which surrounds lthe bodyof said elongated evacuated container and which engages the rim lipportions of said central passage in each of said cooling fins to form athermally conductive contact between said evacuated container and eachof said cooling fins.

References Cited UNITED STATES PATENTS 671947 Martin 315-3951 21/1967staats 315 39.71X

t `Us. C1. xn.

313-46, 15s; sis-39.53, 39.71

